Atrial fibrillation (AF) is the most common arrhythmia and affects over 2 million Americans. AF is a major public health burden as it is associated with a five-fold increased stroke risk, a tripling in heart failure risk and nearly two-fold increase in mortality. There is increasing evidence of a genetic component to AF, and mutations for AF have been described in ion channels and signaling molecules, but such mutations are rare. Genome-wide association studies (GWAS) have uncovered many common variants underlying risk for a wide range of diseases. In 2007, a GWAS for AF in Icelanders identified a susceptibility region for AF on chromosome (chr) 4q25, findings that were replicated in our population with lone AF.1 While the chr 4q25 locus is in important development in our understanding of AF, these findings were based on an initial GWAS of only 550 individuals with AF. Since there are likely many other genetic variants for AF, Drs. Ellinor, Benjamin and Heckbert organized the CHARGE-AF consortium consisting of investigators from 12 studies with over 8,000 subjects with AF and 86,000 subjects without AF. Subjects with lone AF and no evidence of structural heart disease have a particularly high familial aggregation of AF. In preliminary work, we performed a GWAS of lone AF using cases from five studies; a total of 1,335 cases with lone AF and 12,844 referent subjects were available. At chr 4q25, 77 single nucleotide polymorphisms (SNPs) had P<5x10-8. A second, novel locus was identified on chr 1q21; the most significant SNP, rs13376333, is intronic to the potassium channel KCNN3. In a meta-analysis of the primary and replication cohorts, rs13376333 had an odds ratio of 1.52 (P=1.8x10-21). KCNN3 is a member of a family of voltage-independent calcium-activated potassium channels expressed in a number of excitable tissues including the brain40 and the heart;41, 42 however, the role of these channels in the heart is less clear. We propose to extend our work through the following specific aims: Aim 1 - Determine if genetic variation in KCNN3 is associated with AF risk by fine mapping the KCNN3 locus, correlating left atrial RNA levels with KCNN3 genotypes, and determining if a polyglutamine repeat in KCNN3 is associated with AF. Aim 2 - Identify and characterize mutations and rare variants in KCNN3 in subjects with AF. Aim 3 - Identify conserved, non-coding regulatory elements associated with KCNN3 function. Aim 4 - Characterize the cardiac phenotypes of two mouse lines with alterations in KCNN3 function. We believe that our multidisciplinary translational approach integrating available GWAS, cellular electrophysiology, and animal model systems, is uniquely suited to pursue this project. Understanding the molecular determinants of AF will provide insights into the pathogenesis of AF, and eventually provide targets for new therapies to prevent and treat this morbid arrhythmia.